how to change the location of all keyframes in blender timeline at once
Use the Graph Editor:

Enable the Only Include Channels option (little arrow button) so that you'll only see the curve related to the selected object.
Select your object in the 3D view, unfold the tracks in the Graph Editor.
Lock all the locks. Only open the one you want. Or select the track you want and ShiftH to hide all the others. Or use the search field to only display the wanted tracks.
Select the curve and move it manually or with the value you want, then Enter. To move the curve vertically press GY, horizontally press GX